It is often necessary in semiconductor processing to fill high aspect ratio gaps with insulating material. This is the case for shallow trench isolation (STI), inter-metal dielectric (IMD) layers, inter-layer dielectric (ILD) layers, pre-metal dielectric (PMD) layers, passivation layers, etc. As device geometries shrink and thermal budgets are reduced, void-free filling of narrow width (e.g. less than 0.13 μm gap width), high aspect ratio (AR) features (e.g., AR>6:1) becomes increasingly difficult due to limitations of existing deposition processes.
High density plasma (HDP) chemical vapor deposition (CVD), a directional (bottom-up) CVD process, is the method currently used for high aspect ratio gapfill. HDP CVD deposits more material at the bottom of a high aspect ratio structure than on its sidewalls. It accomplishes this by directing dielectric precursor species downward, to the bottom of the gap while simultaneously removing deposited material from the trench top through sputtering by the use of biased RF power applied to the substrate. Thus, HDP CVD is not an entirely diffusion-based or isotropic process.
However, HDP CVD gapfill results in the formation of cusps, also known as overhangs, at the entry region of the gap to be filled. These formations result from sputtering and redeposition processes. The directional aspect of the deposition process produces some high momentum charged species that sputter away material from within the gap. The sputtered material tends to redeposit on the sidewalls of high AR structures. As a result, the entry region of a high aspect ratio structure may close before bottom-up fill has been completed, leaving voids or weak spots within the structure. This phenomenon, known as “pinch-off,” is exacerbated in narrow features. The overhangs cannot be totally eliminated because non-directional reactions of neutral species and sputtering and redeposition reactions are inherent to the physics and chemistry of the HDP CVD processes.
Furthermore, as aspect ratios increase, the shape of the gap itself can contribute to the problem. High aspect ratio gaps often exhibit reentrant features, which make gap filling even more difficult. The most problematic reentrant feature is a narrowing at the top of the gap, wherein the side-walls slope inward near the top of the gap. For a given aspect ratio feature, this increases the ratio of gap volume to be filled to gap entry area seen by the precursor species during deposition. Hence voids or weak spots become even more likely.
In addition to undesirable formations inside the feature, a peak of dielectric material often called a “top hat” forms on the top surface of the substrate on either side of the features. Top hats are deposits of material in the shape of a peak that slopes downwards towards the entry to the gap. If not removed, the top hats are an additional source of redeposition species that increase the rate of overhang growth, thereby effectively making the aspect ratio of the gap even higher.
In some gap fill applications, particularly in the case of small features with high aspect ratios, a multi-step deposition/etch back process has been used in order to remove the overhangs, reduce the top hats, and facilitate void-free gap fill. For example, a deposition and etch process utilizing HDP CVD deposition and an aqueous HF dip for the etch back step has been used. However, this requires that the wafers be cycled between the plasma deposition system and the wet etch back system for a number of cycles. This results in a long cycle time and correspondingly large capital investment to run the multiple steps for gap fill.
In-situ multi-step plasma deposition/etch processes have also been used to keep the entry to the gap from closing before it is filled. Such in-situ HDP CVD deposition and etch back processes are described, for example, in U.S. Pat. Nos. 7,163,896, 6,030,881, 6,395,150, and 6,867,086, the disclosures of which are incorporated herein by reference for all purposes. Some of these in-situ plasma etch back processes use high-energy ions to create a significantly anisotropic sputter etch. Other in-situ plasma etch back processes use chemically-reactive etch gases (e.g., nitrogen trifluoride, NF3) to create a significantly isotropic plasma etch.
In many instances, these sputter etch and reactive plasma etch processes damage the underlying structure of the features on the substrate. It should be noted that once the gap is re-opened by the etch process, the oxide fill material on the sidewalls and at the bottom of the trench is also exposed to the etch reaction. For instance, if a significantly isotropic plasma etch step is used, the upper sidewalls of the structure may be eroded and the etch step may remove nearly as much material as was deposited in the previous etch step. If the underlying structure is eroded via one of these chemical or physical pathways, it can result in compromised device performance (such as higher leakage current). Thus it is desirable to reduce erosion of the underlying structure during the gap fill operations.
Device designs typically have regions of differing feature density on the substrate. There may be one or more regions with high density of features and one or more regions with isolated features. These regions of differing feature density respond to deposition and etch processes differently, often resulting in varying degrees of gap fill. Regions having a high density of features are more susceptible to pinch-off because of their relatively higher aspect ratios. Thus the dense features generally define the maximum amount of deposition before requiring an etch step. They may similarly also define the amount of etch necessary to remove the overhangs at the entry of the gap and reopen the gap. The amount of etch required to remove cusp material from regions having high density of features may however result in over-etch of regions having isolated features. This can result in severe damage to the underlying structure in isolated features. Reduced etching would protect the isolated features but would increase the number of deposition/etch operations necessary to completely fill a gap or in more severe cases void formation could result. Thus, it is desirable to develop more advanced etch processes that can remove the overhang material without damaging the underlying features across regions with dense and isolated features.
While these in-situ multi-step deposition and etch back processes have improved high aspect ratio gap fill capabilities, dielectric deposition processes that can reliably fill high aspect ratio features of narrow width, particularly very small features (e.g., less than about 0.1 μm gap width) with aspect ratios of about 6:1 or more, without leaving voids, continue to be sought. It is even more difficult to uniformly fill high aspect ratio features on a substrate that contains regions of differing feature density. A method to reliably fill both dense and isolated features with fewer operations and without causing detrimental damage to the underlying structure is still needed.